Effect of crosslinking points on bubble nucleation in the microcellular foaming of thermosets

Thermoset foams are often produced by chemical foaming with control of both curing degree and foaming gas generation rate in a heating process. In this study, to analyze the effect of the degree of curing on bubble nucleation independently from the effect of the gas generation rate, batch physical foaming was conducted with a thermoset polymer with controlled crosslinking degree using supercritical carbon dioxide. A styrene/divinylbenzene (DVB) mixture was polymerized with various copolymer ratios and used as model thermoset polymer with different crosslinking degrees. The foaming experiments clearly showed that not the viscosity of the polymer but the distance between crosslinking points was the key factor in determining the bubble nucleation in the physical foaming process. When the distance between the crosslinking points was shorter than the critical diameter of a bubble calculated from classical nucleation theory, bubbles could not be nucleated, i.e., the distance between the crosslinking points determined the foamability of the thermoset polymer.

Automated summary

This study conducted batch physical foaming experiments with a thermoset polymer by controlling the crosslinking degree using supercritical carbon dioxide, and found that the distance between crosslinking points, rather than the viscosity of the polymer, was the key factor determining bubble nucleation in the physical foaming process.